Preview size of the PDF files in the article would vary for browsers. However you can configure the desired width of the PDF file.

Solution

To enlarge the pdf perview:
1_Screenshot-Enlarge_the_pdf_preview_in_the_article.png

  1. Go to Settings -> Knowledge base site -> Custom CSS & Javascript, and Custom CSS tab is the default selection
  2. Paste the below CSS snippet in the window
.content_block .content_container .content_block_text p * {width: 100%;}
  1. Click Save

Outcome

2_Screenshot-Outcome.png